In general, air-conditioners, refrigerators, show cases, and the like, include a refrigerating cycle system. The refrigerating cycle system includes a compressor for compressing a refrigerant, a condenser for condensing the compressed refrigerator to release heat, an expansion valve for lowering the pressure of the condensed refrigerant, and an evaporator for evaporating the refrigerant which has passed through the expansion valve to absorb external heat.
The compressor, the condenser, the expansion valve and the evaporator are connected by a connection pipe, forming a cycle.
In the refrigerating cycle system, as the compressor is operated upon receiving power, high temperature and high pressure refrigerant discharged from the compressor sequentially passes through the condenser, the expansion valve and the evaporator, and then sucked into the compressor, and this process is repeated. In this process, heat is generated from the condenser and the evaporator absorbs external heat to form cold air.
The refrigerator and the show case keeps food items freshly using the cold air generated from the evaporator of the refrigerating cycle system. The air-conditioner selectively circulates heat generated from the condenser of the refrigerating cycle system and cold air formed by the evaporator in a room to maintain the indoor space in an agreeable state.
The refrigerator and the show case are constantly used regardless of the change of the season, while the usage degree of the air-conditioner differs according to seasons.
That is, the usage degree of the air-conditioner is high during summer while it is low in the spring and autumn. Thus, because the usage degrees of the air-conditioner differs according to the summer, spring or autumn, power consumption of the air-conditioner can be considerably reduced by properly changing the operation mode of the air-conditioner.
In particular, recently, the oil prices rise due to the increase in the amount of oil usage worldwide, research and development of an air-conditioner that can minimize power consumption is a key subject. The reduction in power consumption of the air-conditioner would lead to a minimized environment problem.
The refrigerating capacity of the refrigerating cycle system is determined by a compression capacity of the compressor in compressing the refrigerant, and as the compression capacity of the compressor is large, the amount of cold air formed by the evaporator of the refrigerating cycle system is increased, and as the compression capacity of the compressor is small, the amount of cold air formed by the evaporator is reduced.
Thus, in order to effectively operate the refrigerating cycle system, if much cold air is required, the compression capacity of the compressor should be increased, while if a small amount of cold air is required, the compression capacity of the compressor should be reduced.
FIG. 1 is a schematic view showing a general compressor. As shown in FIG. 1, generally, the compressor includes a motor mechanism (motor) for converting electrical energy into kinetic energy and a compression mechanism compresses a refrigerant upon receiving turning force of the motor mechanism.
One of methods for varying the compression capacity of the compressor may be varying the number or rotations of the motor mechanism (motor).
However, if the number of rotations of the motor mechanism is varied, the motor mechanism is high-priced to increase the unit cost of the compressor, resulting in that the competitiveness of the product is degraded.
Thus, research for varying the compression capacity of the refrigerant in the compression mechanism while the number of rotations of the motor mechanism that generate a driving force for compressing the refrigerant is maintained to be uniform is ongoing.
However, the compressor having such a capacity-variable structure as shown in FIG. 1 is operated in a power mode and a power saving mode by using a single capacitor with a single capacitance. Thus, as shown in FIGS. 2 and 3, the compressor has good efficiency in the power mode compared with that in the power saving mode.
This is because, a constant (steady)-speed motor has a better efficiency at an output point in the power mode than at an output point in the power saving mode. Namely, the constant speed motor is not an inverter motor, so it cannot obtain equal efficiency at various output points.
FIG. 2 is a graph showing the relationship between a magnetomotive force (MMF) and a phase angle when the compressor is operated in the power mode, and FIG. 3 is a graph showing the relationship between an MMF and a phase angle when the compressor is operated in the power saving mode. In other words, the motor can have the highest efficiency when the MMF of the main coil (or main winding) and that of the sub-coil (or sub-winding) of the motor are the same (MMF ratio=1) and when the phase angle of the main coil and the sub-coil is a right angle (90°).MMF=N*I   [Equation 1]
wherein ‘N’ is the number of windings, and ‘I’ is current flowing across the coils.
The MMF is a force as a motive power that generates a magnetic flux in a magnetic circuit, which corresponds to an electromotive force in an electric circuit. The size of the MMF is indicated as a force when a unit normal polarity turns full circle according to magnetic circuits.